Look in the Yellow pages under Bicycles and see how many shops are in there. Some shops have a large listing that includes the brands of bikes they sell. Look for Trek.
You can also go to Trek's website and look for dealers in your area. Once you have that information, get a pad of paper start calling the dealers. Tell them that you are interested in buying a Trek 820, and that you would like to know how much it costs. While you're talking, write down the name of the dealer, the cost of the bike, and the name of the person you're talking to. Then thank them for the information and repeat this process for all of the dealers.
When you learn which dealer has the lowest price, call the dealer with the second lowest price, and ask them if they're able to beat the lowest dealers price. If they beat it, then call the lowest price dealer and do the same thing with them. When dealers are no longer willing to budge, then you know that you getting the bike at the lowest possible price.
This may feel uncomfortable for you to do, but it is a skill that is worth learning. You will use it for the rest of your life.